1. Field of the Invention
The present invention relates to a DC motor, for example a motor in which the amount of electric currents to be applied to exciting coils is controlled by means of electromagnetic detectors such as Hall elements or devices, and more particularly to an improvement in the output control for said Hall elements.
2. Description of the Prior Art
A brushless motor is known as one type of DC motors in which Hall elements are utilized for detecting the position of a rotor composed of a permanent magnet.
In a prior brushless motor, drive coils are positioned on a stator, and the Hall elements are usually provided at the same or 180.degree. opposed phase positions relative to the coils to detect the position of the rotor. More specifically the drive currents to be applied to the drive coils are switched in accordance with output signals from a pair of Hall elements having a relative phase difference of 90.degree., to thereby rotate the rotor. FIGS. 1A and 1B are perspective views showing an example of the conventional brushless motor, wherein FIG. 1A shows an external type rotor and FIG. 1B shows a stator to be positioned therein. In FIG. 1A there are shown a shaft 1, a rotor yoke 2 and a permanent magnet 3 constituting the rotor. In FIG. 1B there are shown drive coils 4 of 4-pole structure, and a stator substrate 5 on which a stator yoke having the above-mentioned drive coils and sensors or Hall elements 6, 6' are fixed as illustrated. The stator is inserted into the lower part of the rotor shown in FIG. 1A to constitute the brushless motor.
FIG. 2 is a schematic view showing the internal structure of a conventional brushless motor with an internal type rotor, wherein Hall element sensors 6, 6' are fixed at the same phase positions as two poles 7d and 7a of 4-pole stator coils 7a-7d. As explained in the foregoing, in the conventional brushless motor, the sensors 6, 6' for detecting the rotor position are inevitably positioned close to the stator coils and are therefore easily influenced magnetically by the excitation of the drive coils, so that the detection signals of the rotor position are perturbed, causing changes in the conduction angles of the current waveforms to the drive coils.
FIG. 3 shows a part of a conventional drive circuit for the brushless motor utilizing Hall elements H1, H2. FIG. 4 shows the output waveforms of the Hall elements of FIG. 3 in normal operation, wherein the parts (A) and (B) respectively show the output waveforms of said Hall elements H1, H2. Curves in the part (D) show the output waveforms in a state where the Hall elements provide mutually equal outputs, in which each of the exciting coils L1-L4 is energized during a constant electrical angle of 90.degree. with a constant drive current waveform, as shown in the part (C).
However, when the Hall elements H1, H2 are located close to or at the same phase positions as the exciting coils, as shown in FIG. 1 or FIG. 2, the output signals from the Hall elements H1, H2 may become unbalanced as shown in FIG. 5 (A) due to the influence of heat from the coils. Such unbalance leads to different conduction angles of the drive current waveforms for the exciting coils L1-L4, as shown in FIG. 5 (B), thus resulting in uneven or reduced torque and undesirable vibrations.
Although it is already proposed to position the Hall elements apart from the exciting coils in order to avoid the thermal influence thereof, such arrangement inevitably enlarges the motor structure, hindering the size reduction of the motor and the improvement in the winding density of exciting coils.